1. Field of the Invention
This invention relates to a device for positron-computed tomography, and more particularly to a positron-computed tomograph device provided with a mechanism capable of suppressing the noise component contained in the attenuation correction data obtained by irradiating a given subject with gamma rays for attenuation correction.
2. Description of the Prior Art
In positron-computed tomography (PCT), which is one form of emission-computed tomography (ECT), for the purpose of obtaining a reconstructed cross-sectional image with good quantitative properties on the basis of the distribution of a radio isotope within the subject, two gamma rays (annihilation gamma rays) which are emitted in diametrically opposite directions at the time of annihilation of a positron within the subject are coincidently counted (coincidence). The data obtained by this coincidence requires attenuation correction. Several different methods have been proposed for this attenuation correction. One of these methods (N. Nohara, et al. "POSITOLOGICA: A Positron ECT Device with a Continuously Rotating Detector Ring", IEEE Transactions on Nuclear Science, Vol. NS-27, No. 3, pp. 1128-1136) entails use of a positron-computed tomograph device incorporating, as an emission source for attenuation correction, a gamma ray source formed of positron-emitting radionuclides and effects desired attenuation correction by rotating this source within a desired plane section around the subject while measuring the transmittance of annihilation gamma rays through the subject and computing the attenuation correction based on the data obtained by the measurement (hereinafter referred to as "attenuation correction data").
Even by the method of attenuation correction described above, since the absorption of gamma rays by the subject is still heavy, there is an inevitable disadvantage that the measured coincidence rate obtainable with the emission source of the intensity generally available for attenuation correction falls short of reaching the desired level.
Consequently, either allowance of an ample time for the measurement for the purpose of suppressing the statistical noise present in the data or the use of data including heavy statistical noise in the subsequent processing is inevitable.
To be more specific, as is generally known, the measured coincidence will always include a noise component due to accidental coincidence and scattered coincidence in addition to the true coincidence (coincidence in the form of signals). The rate of true coincidence increases in direct proportion to the intensity of the emission source for attenuation correction under the conditions which render the dead time of the circuit negligible. By contrast, the rate of accidental coincidence increases in direct proportion to the square of the intensity of the emission source. Thus, the ratio of signal to noise in the measured data is degraded in proportion as the radioactive intensity of the emission source increases. By measuring the accidental coincidence by the method of delayed coincidence and subtracting the result of this measurement, therefore, the measured data may be freed from the deviation due to the noise. When this method of correction is carried out, however, the statistical noise in the data tends to increase. For this reason, it is unwise to increase excessively the intensity of the emission source for attenuation correction and it is difficult to obtain transmittance data of a low noise level in a short length of time.
As a measure to lower the rate of accidental coincidence, one might consider disposing lead shields one each at the opposite sides of the emission source for attenuation correction which is revolved around the space intervening between the subject and a plurality of scintillation detectors circumferentially spaced around the subject so as to minimize the effect of the emission source exerted upon the detectors other than those falling in the direction of measurement. Even by this method, the accidental coincidence is inevitably measured among the detectors. Thus, the suppression of noise by this method has its limit.
In the circumstance, development of a PCT device which is capable of suppressing the noise component of the data for attenuation correction, enhancing the resolvability, and producing a reconstructed image of excellent quality has been desired. Further, development of a technique which enables a mechanism capable of performing such advantageous attenuation correction to be adapted for various devices such as, for example, a device utilizing a wobbling scanner, a device for straight-rotary composite scanning (C. W. Williams, et al: IEEE, Vol. NS-28, No. 2, pp. 1736-1740, 1981), a device utilizing a dichotomic scanner (Z. H. Cho, et al: IEEE, Vol. NS-28, No. 1, pp. 94-98, 1981), and a device utilizing non-scanning (S. E. Derenzo, et al: IEEE, Vol. NS-28, No. 1, pp. 81-89, 1981) has been in demand.